Autonomous vehicles, such as vehicles that do not require a human driver, can be used to aid in the transport of passengers or items from one location to another. Such vehicles may operate in a fully autonomous mode where passengers may provide some initial input, such as a pickup or destination location, and the vehicle maneuvers itself to that location.
In order to do so safely, these vehicles must be able to detect and identify objects in the environment as well as respond to them quickly. This is especially true in situations involving emergency vehicles. To detect such vehicles using visual cues can be extremely difficult, especially as these vehicles can differ dramatically, and in many situations may actually resemble non-emergency vehicles. Moreover, if flashing lights are not readily discernable by the autonomous vehicle's perception system, such as when an emergency vehicle is occluded or out of range of the vehicle's perception system, detection can be nearly impossible. One common feature of emergency vehicles are the use of sirens to signal an emergency or a need for the emergency vehicles to pass through traffic quickly. However, in some cases, an emergency vehicle may not be directly observed or identified, even where its presence is identified through its siren. In such cases, it is critical for the autonomous vehicle's reaction that the inferring the emergency vehicle's position, bearing, velocity, and intent through detection and tracking of the siren.